docs: def __get_default_currency(self):
          """Read the default currency from GnuCash preferences"""
          # If we are on Windows, read from registry.
          if sys.platform == "win32":
              # read from registry
              def_curr = self.book["default-currency"] = self.__get_default_currency_windows()
          else:
              # return the currency from locale.
              # todo: Read the preferences on other operating systems.
              def_curr = self.book["default-currency"] = self.__get_locale_currency()
  
          return def_curr